Schlieren Photography

The detail behavior of mixture formation during ignition delay period was investigated using the schlieren photography system with a high speed digital video camera. The schlieren images of spray evaporation and mixture formation without ignition were detected with this system. This method can capture spray evaporation, spray interference and mixture formation clearly with real images. The optical equipments of schlieren photography are also shown in Fig.5. The schlieren monochromes images were captured by a high-speed digital video camera (Eastman Kodak Ektapro, HS4540) with frame speed of 13500 fps. In conducting these tests, special lens (Sigma 170-500 mm, F5-6.3 APO) are needed due to the longer of focal length from concave mirror. The surface of injector holder that composed of base surface of the chamber was mirror finished. The principle of schlieren technique is that of introducing a Xe-light source (Ushio XS-102 AA-A) and projected with concave mirror (focal length 2000 mm) passing through the mirror, half mirror and quartz mirror placed in front of chamber.

Here, the straight and equal intensity of light is produced by the Xe-light source. Then, the parallel light of Xe light created by concave mirror was reflected by the mirror surface. A ring edge with a diameter of 2.5 mm was used as a schlieren stop and was set at the focal point of another concave mirror. The atmosphere was filled by nitrogen gas in the case of schlieren photography.

Effects of ambient density

Fig.5(a) shows the schematic diagram of a free-piston type rapid compression machine which used tosimulate actual phenomenon inside the combustion chamber and the basics specifications are listed in Table 2. The charging pressure pc were changed to p_c =100 kPa, 150 kPa and 200 kPa with keeping ambient temperature of T_i = 850 K. At every condition, ambient density ρ was ρ =16.6 kg/m³, 25.0 kg/m³, 33.3 kg/m³, respectively. Schlieren photograph indicated the detail of mixture formation was resulting from non-combustion diesel sprays which avoided by using nitrogen ambient and the direct photography technique is used to capture the flame development images, as shown in Fig.5(b).

Fig.6(a) compares tendencies of flame development images well correspond to the distribution of combustible mixture observed in schlieren images. In particular, at p_c = 100 kPa (ρ =16.6 kg/m³) and 150 kPa (ρ = 25.0 kg/m³), fuel spreads out between each spray and large amount combustible mixture is formed at the time of ignition. At p_c = 100 kPa and 150kPa, flame is observed near the spray centerline because of fuel is continuously injected into the spray centerline even after ignition, creates high temperature and rich atmosphere region. However, pc=150kPa is producing larger area of flame compares to p_c =100 kPa due to less formed of combustible mixture. In contrast, at p_c = 200 kPa (ρ = 33.3 kg/m³) that is high ambient density, weakens and bended the spray penetration due to the swirl motion, fuel is mainly distributed at the center of the combustion chamber and little fuel is distributed near the chamber wall. Moreover, flame images shows the highest luminosity flame develops within center region of the chamber and possible to create locally rich combustion around chamber center. In this chamber, the condition of p_c = 100 kPa seems to produce better distribution of the mixture than the cases of p_c = 150 and 200 kPa.

Fig.6(b) shows heat release rate dQ/dt together with nozzle needle lift NL against time. According to the figure, increasing ambient density produces high heat capacity at spray boundary reflects shortens ignition delay and earlier rise of heat release rate. However, at high ambient density, increasing rate of initial heat release is gentle and combustion duration becomes long as compared with lower ambient density condition. In addition, heat release pattern related with the changes of flame pattern, are compared within the same of time at all conditions. As result, the mixture formation process may affect heat release history and flame pattern as well.